Image sensor arrays are used in many imaging applications, such as digital cameras, scanners, video camcorders and the like. A common sensor array used in conventional imaging applications is a Bayer pattern image sensor array, in which twice as many green pixels as red and blue pixels are used to detect an image. For images that have red frequency content greater than the Nyquist frequency, an alias artifact commonly known as false colors will occur with a Bayer sensor. This artifact typically occurs where there are sharp contrast images, such as shadows that have discoloration along the edges of the high contrast image or where there are large areas of high frequency content, such as cyclone fences and shingled rooftops, resulting in a large area of discoloration.
In order to eliminate or minimize the effects of aliasing, conventional imaging applications filter out the higher frequencies in an image or minimize those frequencies using a lens before the image is projected onto the sensor. For filtering out the higher frequencies, an optical low pass filter (OLPF) is typically used. The OLPF assures that light rays from any spot on an object will be projected onto four adjacent pixels. However, disadvantages associated with the use of OLPFs include their expense and the amount of space they require between the lens and the sensor. The expense and space requirements prohibit the use of OLPFs in low cost cameras and in low profile cameras. Thus, as the size of camera modules decreases by using fewer pixels, the feasibility of using OLPFs in these cameras is virtually eliminated.
An alternative to the use of OLPFs is the use of a lens designed to act as a filter by reducing the level of the higher frequency content in an image. However, if frequencies above the Nyquist frequency for a particular lens are not sufficiently filtered out, false colors and moiré patterns will occur. If they are filtered to an acceptable level, false colors may still appear with an attenuated intensity that is considered acceptable. One problem with this alternative, however, is that the lens must be designed to be less sharp than would otherwise be desired in order to filter out the higher frequencies, resulting in a loss of resolution.
As an alternative to Bayer pattern image sensor arrays, multicolor pixel sensor arrays have been developed recently. A pixel is one element of space that provides one piece of information about an object in space. Thus, a multicolor pixel sensor array has been described as “seeing” the same number of pixels in space as a Bayer pattern image sensor array. However, as described above, typical imaging applications that use Bayer pattern image sensor arrays, or other single color pixel sensor arrays, blur the light coming from an element of an object onto four pixels of the sensor array. Because of this, these imaging applications are only able to “see” a picture element that is four times the size of a picture element that could be “seen” if a perfect lens focused on one single pixel of the sensor array.
An OLPF is generally used to assure that the light from a picture element will be cast on all four colors. For example, for a Bayer pattern image sensor array, the light from a picture element is cast on Green on the Red Channel, Green on the Blue channel, the Red channel, and the Blue channel, which make up Gr, Gb, R and B. As an alternative to an OLPF, a lens may be used to blur the light from a picture element on all four colors (Gr, R, Gb and B). Thus, a single color pixel sensor array is unable to “see” a picture element for each color sensor.
For a Bayer pattern image sensor array, false colors will occur if an OLPF is not used or the light coming from an object is not blurred over four colors, i.e., Gr, R, Gb and B. The colors of cyan and yellow will occur along the edges of images or areas of high frequency. It has been found that false colors are very objectionable to the end user. For this reason, an OLPF or the practice of blurring the image is used.
However, this is not the case with moiré patterns. Moire patterns are the result of aliasing, which will occur when the sampling frequency of the array is exceeded. In order to prevent moiré patterns, the special frequencies reaching the array at the Nyquist frequency must be reduced. It has been observed that the end user is less sensitive to some low level of moiré patterns. Oftentimes, end users using low cost cameras will not object, but end users using higher cost cameras will not want to see moiré patterns. This means a camera manufacturer can balance the use of a sharper lens against what they think is an acceptable level of moiré patterns. Thus, while false colors are a dead stop and must be greatly minimized or eliminated, end users are more tolerant of moiré patterns.
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